Abstract
Previously proposed theories on the minimum-heat-flux-point (MHF-point) condition were examined using available data obtained from the immersion cooling experiments of spheres in water. The sphere diameter ranged from 9.5 mm to 30mm and the liquid subcooling from 0 K to 85 K. The limiting liquid superheat predicted by the Lienhard equation was compared with the wall superheat at the instant of liquid-solid contact at the MHF-point. The results showed that the wall superheat at the MHF-point was not limited by the limiting liquid superheat and its value was connected with the collapse mode of vapor film. The collapse mode was a coherent collapse at a low wall superheat and the mode changed to a propagative collapse as the wall superheat increased. The critical vapor film thickness obtained from the linear stability analysis of vapor film was compared with the calculated value of average vapor film thickness at the MHF-point. For all data, the ratio of the average vapor film thickness and the critical vapor film thickness was correlated well as a function of liquid subcooling. The ratio decreased with increasing liquid subcooling and tended to 0.8 to 1 depending on the experiments. This indicated that the MHF-point at a high liquid subcooling was determined by the critical vapor film thickness. A physical consideration was given to the effect of liquid-solid contact that occurred in the film boiling region on the calculated value of the vapor film thickness and the stability of vapor film.
